1. Field of the Invention
The present invention relates to a pneumatic motorcycle tire, which is provided with a spiral belt and a pair of axially spaced reinforcing layers radially inside of the belt to improve the cornering stability and straight running stability.
2. Description of the Background Art
Recently, as the expressway network has become widely developed, the demand for high speed running of motorcycles is increasing.
The radial tire for such motorcycles was hitherto designed and manufactured on the basis of a radial tire for a four-wheel vehicle, and it could not follow up the cornering performance and straight running performance which are characteristic of a motorcycle. In particular, high speed steering stability was inferior.
As a reason therefor, a conventional belt 1 for reinforcing the tread portion was disposed in the same way as in the four-wheel vehicle, with inclining its cords (b) at 13 to 30 degrees to the tire equator as shown in FIG. 7(A). Such belt (a) with the inclined cords (b) was, as shown in FIG. 7(B), formed by
cutting a sheet (d) of tire fabric having cords (b) extending in the longitudinal direction thereof, by the same angle as the inclination angle (alpha) of the cords (d) of the belt (a), to form a parallelopiped pieces (e), and
linking them in a series, with abutting the edges (f) of the pieces (e). Thus, the formed belt (a) has abutting parts (f) passing obliquely to the tire axial direction. As the abutting part does not contact with the ground simultaneously during running, the handling becomes unstable, and the straight running performance is impeded. Further, during cornering, handling is also unstable. These occur particularly during high speed running.
In order to solve part of these problems, a tire having a belt formed by spirally winding a narrow and long ribbon-shaped strip has been proposed. In this tire, as shown in FIG. 8, one ribbon-shaped strip (g) is wound continuously from one end to the other end of the belt (h). Therefore, the belt (h) provides a uniform rigidity or a constant reinforcement for both the tread central region (j) and shoulder region (k).
On the other hand, as a motorcycle is inclined largely during cornering unlike the four-wheel vehicles, a larger camber thrust must be generated during cornering.
The above-mentioned tire having the uniformly reinforced tread portion can be improved in the straight running performance, but the tread portion is liable to collapse by the lateral force applied during cornering, and the cornering performance is deteriorated. This phenomenon is particularly notable when the belt is formed by spirally winding a ribbon-shaped strip. As the ribbon-shaped strip is wound spirally, the cords in the ribbon-shaped strip are wound at nearly 0 degrees to the tire equator. As a result, the resistance of the carcass and belt against lateral force is poor.
In order to increase the resistance against lateral force, if a reinforcing cord layer (m) made of a sheet of conventional tire fabric is disposed over the entire width of the belt (g) as shown in FIG. 9, the rigidity of the tread shoulder region is improved, and the camber thrust occurs and becomes a large value early when the motorcycle makes a turn, whereby the cornering performance can be improved.
However, the ground contact and steering stability during straight running are deteriorated since the tread central region (j) is increased in the bending rigidity.